Interdecadal and Interannual Variability of Winter Precipitation in Southeast China

Interdecadal variability of observed winter (DJF) precipitation in Southeast China (1961 to 2010) is characterized by the first EOF of the three-monthly Standardized Precipitation Index (SPI) subjected to a 9-year running mean, while the differences from the original describe the interannual fluctuations. For interdecadal time scales the dominating spatial modes represent monopole features over Southeast China involving the Arctic Oscillation (AO) and the sea surface temperature (SST) anomalies over western Pacific. Dynamic composite analysis (based on NCEP/NCAR and ERA-40 reanalyzes) reveals the following results: (i) Interdecadal SPI-variations show a trend from a dryer state in the 1970s via an increase during the 1980s towards stabilization of wetter conditions commencing with the 1990s. (ii) In mid-to-high latitudes the weakened southward flow of polar airmasses induces low-level warming over Eurasia due to stronger Arctic Oscillation (AO) by warmer zonal temperature advection. This indicates that the precipitation increase in Southeast is attributed circulation anomalies over mid-to-high latitudes which are related to AO. (iii) The abnormal moisture flux along the southwestern boundary of the abnormal anticyclone over south Japan (and its anomalous south-easterlies) is modulated by the sea surface temperature (SST) anomalies over Western Pacific; a positive (negative) SST anomaly will strengthen (weaken) the warm and moist air flow, leading to abundant (less) precipitation in Southeast China. This demonstrates the collaborative effect of AO and SST anomalies in determining the nonlinear trend observed in winter precipitation over Southeast China. For interannual time scales the dominating spatial pattern also represents monopole patterns. Composite analysis (with resampling test) of the associated circulation anomalies reveals the following results: (i) The wet (dry) winter is a result of the strengthened (weakened) northward warm moist air over east coast of China, which is caused by the weakened (strengthened) East Asian Winter Monsoon (EAWM) due to weakening (strengthening) of Siberia High (SH) and eastward (westward) extending of East Asian Trough (EAT). (ii) The effects of El Niño and sea surface temperature (SST) anomalies over South China Sea (SCS) on rainfall in Southeast China are independent. El Niño years, the anomalous anticyclone (cyclone) over Philippines leads to positive (negative) anomalies of rainfall over South China, while in years with the anomalous positive (negative) SST over SCS (non-ENSO), more (less) water vapor is conveyed to Southeast China, thereby enhancing (reducing) precipitation over south of the Yangtze River. (iii) Contributions from all impact factors (EAWM, SH, EAT, El Niño events and SST SCS anomalies) do not counteract with one aother to generate the Southeast China winter precipitation variability.